The present invention relates generally to multilayer blown films made from polyolefins and methods to make the films.
Since it was first invented, 3-layer blown film coextrusion has improved the mechanical properties of films tremendously relative to films made with the same composition in a monolayer construction. More recently, the limit of maximum achievable mechanical properties using 3-layer films appears to have been reached in many specific applications using the latest polyolefin resins.
Five-layer blown films have been in existence for many years, but these were typically used to create films which contained resins which were non-polyolefinic with the intention of creating barrier properties. The dies used for this technology have very thin core layers because the resins typically used for the barrier layer are relatively expensive and their use was desirably minimized by employing a thin layer. A layer distribution which emphasized thick skin layers and thin sub-skin and core layers was typical for these types of dies. Polyolefin-only films made on such equipment did not show major improvements in properties over equivalent composition films made in conventional 3-layer equipment.
There is a need in the art for films that improve on the mechanical property limits achievable with 3-layer blown polyolefin films. There is also a need in the art to improve the mechanical properties of blown films having more than three layers.